Inductor utilizing pad metal layer

ABSTRACT

An inductor utilizing a pad metal layer. The inductor comprises a metal spiral, a metal bridge, and a metal interconnect. The metal bridge is formed with the pad metal layer and a plurality of vias and has one end connected to the metal spiral. The metal interconnect is connected to the other end of the metal bridge. In addition, resistivity of the pad metal layer is lower than that of the metal spiral.

The present application is a divisional of U.S. patent application Ser.No. 11/592,203 filed Nov. 3, 2006, now abandoned the entire contents ofwhich are hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to spiral inductors and, in particular, to spiralinductors utilizing a pad metal layer.

2. Description of the Related Art

Spiral inductors are important, performance-limiting components inmonolithic radio-frequency (RF) circuits, such as voltage-controlledoscillators (VCOs), low-noise amplifiers (LNAs), and passive-elementfilters. Quality factor of the inductors, limited by resistive losses inthe spiral coil and substrate losses, is expressed as Q=im(Z)/re(Z),wherein Z is impedance of the inductor.

FIG. 1A shows a layout of a conventional spiral inductor. FIG. 1B is across section of the spiral inductor 100 in FIG. 1A. As shown, thespiral inductor is formed on a silicon substrate using multilevelinterconnects provided in mainstream silicon processes. At least twometal layers are required to form a basic spiral coil 110 and anunderpass 120 to return the inner terminal of the spiral coil to theoutside. The spiral coil 110 is typically formed with a top metal layerand the underpass 120 with a metal layer lower than the top metal layer.The underpass 120 is connected to the spiral coil such that the innerterminal of the spiral inductor 100 is routed out.

The top metal layer in semiconductor process is typically much thickerthan other metal layers and thus the spiral inductor 100 is typicallyformed with the top metal layer such that quality factor thereof ismaximized. Unfortunately, the underpass 120 is typically formed with athinner metal layer, leading to increased resistance of the spiralinductor 100. As a result, the spiral inductor 100 suffers fromresistive loss and quality factor thereof is degraded.

BRIEF SUMMARY OF THE INVENTION

An embodiment of an inductor utilizing a pad metal layer comprises ametal spiral, a metal bridge, and a metal interconnect. The metal bridgeis formed with the pad metal layer and a plurality of vias and has oneend connected to the metal spiral. The metal interconnect is connectedto the other end of the metal bridge. Resistivity of the pad metal layeris lower than that of the metal spiral.

Another embodiment of an inductor utilizing a pad metal layer comprisesa metal spiral, a metal bridge, and a metal interconnect. The metalbridge is formed with a first metal layer and a plurality of vias andhas one end connected to the metal spiral. The metal interconnect isconnected to the other end of the metal bridge. The metal spiral and themetal interconnect are formed with a second metal layer and the padmetal layer disposed on the second metal layer. In addition, the firstmetal layer is lower than the second metal layer.

The invention provides an inductor utilizing a pad metal layer. Sincethe pad metal layer is typically thick, quality factor of the inductorutilizing the pad metal layer is improved. In addition, the inductorutilizing the pad metal layer is compatible with standard process suchthat no process modification is required.

A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1A shows a layout of a conventional spiral inductor;

FIG. 1B is a cross section of the spiral inductor in FIG. 1A;

FIG. 2A shows a layout of an inductor utilizing a pad metal layeraccording to an embodiment of the invention;

FIG. 2B is a cross section of the inductor utilizing the pad metal inFIG. 2A;

FIG. 3A shows a layout of an inductor utilizing a pad metal layeraccording to an embodiment of the invention;

FIG. 3B is a cross section of the inductor 300 utilizing the pad metallayer in FIG. 3A; and

FIG. 4 is a variation of the inductor 300 utilizing the pad metal layerin FIG. 3B.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carryingout the invention. This description is made for the purpose ofillustrating the general principles of the invention and should not betaken in a limiting sense. The scope of the invention is best determinedby reference to the appended claims.

FIG. 2A shows a layout of an inductor utilizing a pad metal layeraccording to an embodiment of the invention. The inductor 200 comprisesa metal spiral 210, a metal bridge 220, and a metal interconnect 230.FIG. 2B is a cross section of the inductor 200 utilizing the pad metallayer in FIG. 2A. As shown in FIGS. 2A and 2B, the metal bridge 220 isformed with the pad metal layer 221 and a plurality of vias 223 and hasone end connected to the metal spiral 210. More specifically, the padmetal layer 221 is substantially an aluminum layer. The metalinterconnect 230 is connected to the other end of the metal bridge 220.Both the metal spiral 210 and the bridge 220 are typically formed with atop metal layer. More specifically, the top metal layer is substantiallycopper.

In a 0.13 μm semiconductor process, the top metal layer is typicallyformed with a copper layer of 9 KÅ and the metal layers under the topmetal layer typically with copper layers of about 3 KÅ. The pad metallayer is typically formed with an aluminum layer of 12 KÅ withresistivity thereof equivalent to that of a copper layer of 8 KÅ. Inconventional spiral inductors, resistivity of the underpass thereof ismuch higher than the metal spiral. As a result, the conventional spiralinductor suffers from resistive loss and quality factor thereof isdegraded. In the inductor utilizing the pad metal layer according to anembodiment of the invention, the metal bridge has lower resistivity thanthe underpass. As a result, the inductor utilizing the pad metal layerdoes not suffer from significant resistive loss and quality factorthereof is higher than that of the conventional spiral inductor. It isnoted that the inductor can not only be used in 0.13 μm semiconductorprocess but also in semiconductor process of other generations. Thus,thickness of the top metal layer and the pad metal respectively rangesfrom about 1.5 KÅ to 50 KÅ and 5 KÅ to 40 KÅ

FIG. 3A shows a layout of an inductor utilizing a pad metal layeraccording to an embodiment of the invention. The inductor 300 comprisesa metal spiral 310, a metal bridge 320, and a metal interconnect 330.FIG. 3B is a cross section of the inductor 300 utilizing the pad metallayer in FIG. 3A. As shown in FIGS. 3A and 3B, the metal bridge 320 isformed with a first metal layer 321 and a plurality of vias 323 and hasone end connected to the metal spiral 310. The metal interconnect 330 isconnected to the other end of the metal bridge 320. The metal spiral 310and the metal interconnect 330 are formed with a second metal layer 350and the pad metal layer 340 disposed thereon. More specifically, the padmetal layer 340 is substantially an aluminum layer. In addition, thefirst metal layer 320 is lower than the second metal layer 350. Morespecifically, the first metal layer 320 and the second metal layer 350are substantially copper layers.

The metal bridge in the inductor utilizing the pad metal layer is thesame as the underpass in a conventional spiral inductor. Resistivity ofthe metal bridge is higher and results in significant resistive loss ofthe spiral inductor. However, the metal spiral 310 and the metalinterconnect 330 of the inductor utilizing the pad metal layer accordingto an embodiment of the invention have an additional pad metal layer340. As a result, low resistance of the metal spiral 310 and the metalinterconnect 330 results in less resistive loss and quality factor ofthe inductor utilizing the pad metal layer is higher than that of theconventional spiral inductor.

FIG. 4 is a variation of the inductor 300 utilizing the pad metal layerin FIG. 3B. The inductor utilizing the pad metal layer in FIG. 4 issimilar to that in FIG. 3B and only differs in that there are vias VIAcoupled between the pad metal layer 340 and the second metal layer 350.

The invention provides an inductor utilizing a pad metal layer. Sincethe pad metal layer is typically thick, quality factor of the inductorutilizing the pad metal layer is higher. In addition, the inductorutilizing the pad metal layer is compatible with standard process and noprocess modification is required.

While the invention has been described by way of example and in terms ofpreferred embodiment, it is to be understood that the invention is notlimited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements as would be apparent to thoseskilled in the art. Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

1. An inductor utilizing a pad metal layer, comprising: a metal spiral;a metal bridge formed with a first metal layer and a plurality of vias,the metal bridge having one end connected to the metal spiral; and ametal interconnect connected to the other end of the metal bridge,wherein the metal spiral and the metal interconnect are respectivelyformed with a second metal layer and the pad metal layer disposedthereon and the first metal layer is lower than the second metal layer.2. The inductor as claimed in claim 1, wherein the first and secondmetal layers comprise copper-containing layers and the pad metal layercomprises an aluminum-containing layer.
 3. The inductor as claimed inclaim 2, wherein thickness of the second metal layer ranges from about1.5KÅ to about 50KÅ.
 4. The inductor as claimed in claim 2, whereinthickness of the pad metal layer ranges from about 5KÅ to about 40KÅ. 5.The inductor as claimed in claim 1, wherein the pad metal layer is indirect contact with the second metal layer.
 6. The inductor as claimedin claim 1, wherein the pad metal layer of the metal spiral is separatedfrom that of the metal interconnect.
 7. An inductor utilizing a padmetal layer, comprising: a metal spiral; a metal bridge formed with afirst metal layer and a plurality of vias and having one end connectedto the metal spiral; and a metal interconnect connected to the other endof the metal bridge; wherein the metal spiral and the metal interconnectare respectively formed with a second metal layer and the pad metallayer coupled thereto and the first metal layer is lower than the secondmetal layer.
 8. The inductor as claimed in claim 7, wherein the firstand second metal layers comprise copper-containing layers and the padmetal layer comprises an aluminum-containing layer.
 9. The inductor asclaimed in claim 7, wherein thickness of the second metal layer rangesfrom 1.5KÅ to 50KÅ.
 10. The inductor as claimed in claim 7, whereinthickness of the pad metal layer ranges from 5KÅ to 40KÅ.
 11. Theinductor as claimed in claim 7, wherein the pad metal layer is coupledto the second metal layer by a via.
 12. The inductor as claimed in claim7, wherein the pad metal layer of the metal spiral is separated fromthat of the metal interconnect.